Comment # 9 on bug 1204956 from Michael Andres

If this is desired: touch /var/cache/zypp/packages/.no_auto_prune

All clients will need to use at least libzypp-17.31.5 and 'keep-packages' must
be turned on for all repos. The flag will solely prevent unknown/orphaned
package caches from being pruned automatically. 
Explicit requests like 'zypper clean' are not affected. They will clean the
cache as expected.